Benchmark study of potential energies and vibrational levels using the reduced multireference coupled cluster method. The HF molecule

Abstract

Potential energies and vibrational term values for the electronic ground state of the HF molecule are calculated using four-reference reduced multireference (RMR) coupled cluster method with singles and doubles (CCSD). The convergence of the computed vibrational levels with a systematic increase in the size of the correlation-consistent basis sets is explored. The atomic natural orbital (ANO) basis is also used for comparison. The performance of the CCSD and RMR CCSD methods over the entire region of nuclear separations is assessed via comparison with the experimentally determined potential and vibrational energies. It is shown that although the standard CCSD method with a small basis set may lead to good results, the CCSD error increases with the increasing size of the basis set and becomes significant when approaching the complete basis set limit. In contrast, the excellent performance of RMR CCSD is demonstrated by the fact that the computed vibrational levels converge to the experimental results when the size of the basis set employed is systematically increased. For the first 20 vibrational levels, the average error of the four-reference RMR CCSD using the cc-pVQZ basis is 113.8 cm −1, while this error amounts to 1099 cm −1 when the CCSD method is employed. The performance of the perturbatively truncated RMR CCSD method in computations of vibrational levels is also investigated for the ANO basis.